Operating Time of MSPM0 Powered by a Capacitor

Abstract

MSPM0 is typically powered from a 3.3V DC supply with decoupling capacitors. A common situation here is that when the DC power supply is unstable, it is easy to cause the MCU to stop working. To overcome this problem, an external capacitors can be chosen to maintain the power supply. Therefore, it is important to select the appropriate capacitor to in the event of a power failure. In this application note, the operating time of different operating currents and capacitor values is tested, and a simple model relationship between the three is obtained, which, under the known normal operating current, can help to choose the appropriate supply capacitor.

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1 Relationship Between I, C and t

Under certain conditions, capacitors can be used to power the MCU. But how long the capacitor can last, and what factors are related to the maintenance time. Here, the relationship between hold-up time and supply capacitance is given in Equation 2.

Equation 1.

$t = \frac{C \times ({V_{work}}^{2} - {V_{cutoff}}^{2})}{I \times (V_{work} + V_{cutoff})}$

where,

t is the device operating maintenance time,

C represents the capacitance value, which is chosen by the user,

V_work is the normal operating voltage of the MCU, that is, the starting voltage of the capacitor power supply,

V_cutoff is the cut-off voltage at which the MCU stops working. As the power supply is below Brown-out reset voltage level 0 (BOR0-), the device will discharge slowly because only Power-on Reset (POR), Bandgap (BG) and BOR are active, which means the MCU can not work normally. According to device-specific data sheet, the maximum of V_BOR0- is 1.61V, I is the constant operating current of MCU, which depends on the operating mode of the MCU and the operation of the peripherals. In addition to direct measurement, here we provide two ways. One is to consult the device-specific data sheet, which includes the operating current of different peripherals under different operating conditions. The other is to directly use the EnergyTrace™ provided by TI to calculate the operating current of the device.

According to Equation 2, the correspondence between t and C can be obtained with the known operating current. This provides a reference to select the appropriate capacitor.